Three-Dimensional Electrochemical Axial Lithography on Si Micro- and Nanowire Arrays

A templated electrochemical technique for patterning macroscopic arrays of single-crystalline Si micro- and nanowires with feature dimensions down to 5 nm is reported. This technique, termed three-dimensional electrochemical axial lithography (3DEAL), allows the design and parallel fabrication of hybrid silicon nanowire arrays decorated with complex metal nano-ring architectures in a flexible and modular approach. While conventional templated approaches are based on the direct replication of a template, our method can be used to perform high-resolution lithography on pre-existing nanostructures. This is made possible by the synthesis of a porous template with tunable dimensions that guides the deposition of well-defined metallic shells around the Si wires. The synthesis of a variety of ring architectures composed of different metals (Au, Ag, Fe, and Ni) with controlled sequence, height, and position along the wire is demonstrated for both straight and kinked wires. We observe a strong enhancement of the Raman signal for arrays of Si nanowires decorated with multiple gold rings due to the plasmonic hot spots created in these tailored architectures. The uniformity of the fabrication method is evidenced by a homogeneous increase in the Raman signal throughout the macroscopic sample. This demonstrates the reliability of the method for engineering plasmonic fields in three dimensions within Si wire arrays.


Table of content
. Si@metal wire array dimensions.
The dimensions of all the Si@metal wire array synthesized and shown in both the main text and the supporting info are reported here. The smallest dimensions achieved to date in terms of Si diameter (~ 160 nm, Fig. S13), ring height (~ 40 nm, Figure S9), gap length (~ 5 nm, Figure S9), and ring thickness (~ 30 nm, Figure S11) are shown in bold and red. * The Si wire diameter, height and thickness of the ring, and gap length were measured on one Si@Au structure (TEM image).

Materials and chemicals
All chemicals and solutions were used without further processing, unless noted otherwise. After cooling to room temperature the dispersion was filtered and purified by centrifugation and redispersion and dialysis.

Colloidal lithography
The PS colloids (d = 590 nm, d = 1100 nm) were assembled at the air/water interface using ethanol as spreading agent. 1 Prior to the interfacial self-assembly process, the PS colloids Colloids with a diameter of 1.5 µm were obtained from Polysciences. A cleaned, plasma treated c-Si substrate was coated by spin coating (3500 rpm for 3 min) of the 1.5 µm PS dispersion (10 wt%) mixed with 3 parts of methanol : Triton X100 (400:1).
After monolayer deposition, the polystyrene particles were reduced in size to produce a nonclose packed architecture using oxygen plasma (FEMTO, Diener Electronic, 50 W, oxygen flow 4 mL min -1   Au was deposited at -950 mV using Orotemp 24 Rack solution.
Ag was deposited at -940 mV using Cyless solution.
Ni was deposited at -900 mV using a homemade aqueous Ni solution (0. Such well-defined arrays are made possible by self-assembling the PS colloids at the air-water interface, leading to large crystalline defect-free hexagonal arrays with crystals in the mm² range ( Figure S1).

Selective Etching
After the metal deposition, the area covered with the silver paste was cut away using a glass Si@Ni nanowire arrays without a metal base layer ( Figure S16).       The metal shell thickness is adjusted by the number of SiO 2 steps. a-b) Thin gold ring (thickness: 34 ± 10 nm) prepared using three SiO 2 deposition steps (3 hour deposition). a) Schematic of the synthetic pathway based on three SiO 2 deposition step. b) SE cross-sectional SEM image. c-f) Thick gold ring (thickness: 154 ± 33 nm) prepared using 15 deposition SiO 2 steps. c) Schematic of the synthetic pathway to make thick metal shell using thick SiO 2 sacrificial shells. d) SE cross-sectional SEM image. e-f) Top-view BSE images showing homogeneous ring thickness (obtained using the ESB in-lens detector).       a-d) The plane wave source propagates along the y-axis (from top to bottom) and the electric field is polarized along the x-axis. The top of the wire is located at y = 1500 nm, the rings at y = 0 nm and the bottom of the wire at y = -1500 nm. a-c) E-field intensity enhancement maps of the Si@Au nanowires. Identical rainbow log scale for the three maps. a) Single Si nanowire. b) Si@Au ring nanowire. c) Si@Au ring dimer nanowire. d) Linescan of the Efield intensity along the center of the Si@Au nanowires (x=0, z=0) in the longitudinal direction from top to bottom, as indicated by the black arrows in a-c). Linear scale. Black curve: Si wire (no Au ring), blue curve: Si@Au ring wire and red curve: Si@Au ring dimer. The Au ring increases the amount of light reaching the top half of the Si wire (above the rings), while suppressing it in the bottom half (below the ring) in comparison to the nanowire with no gold rings. The effect is even more pronounced in the case of the Au ring dimers.